The present invention generally relates to optical signal processing apparatuses and especially to an optical time-division multiplex signal processing apparatus.
Optical time-division multiplexing technology is an indispensable art in today's super-fast optical telecommunication system in addition to the technology of wavelength-multiplexing.
In the optical time-division multiplexing technology, signal components of channels are sampled with different timings. By superposing the signal components thus sampled, a multiplexed optical signal is formed. As a result, it becomes possible to transmit optical signals of plural channels through a single optical fiber.
In the technology of optical time-division multiplexing, the speed of incoming optical time-division multiplex signals is generally much higher than the response speed of a photodiode or other high-speed optical detectors. Because of this, it is practiced to first separate the incoming optical time-division multiplex signal supplied to a receiver into optical signal components of respective channels by using an electro-optic modulator and then detect the optical signal components thus separated by a photodiode.
FIG. 1 shows an example of a conventional optical time-division multiplex signal receiver 10.
Referring to FIG. 1, an optical time-division multiplex signal OTDM transmitted through an optical fiber 11 is supplied to an electro-optic modulator 12. The electro-optic modulator 12 is further supplied with a clock voltage signal corresponding to a desired channel from a clock signal source 13 and changes the transmittance thereof in response to the clock voltage signal. As a result, the optical sampling is achieved in the electro-optic modulator 12 with respect to the incoming optical time-division multiplex signal with the timing provided by the clock voltage signal, and an optical signal component for a specified channel is extracted. The optical signal component thus extracted is then supplied to a photodiode 14 through an optical fiber or an optical waveguide for conversion to an electric signal.
FIG. 2 shows the construction of another optical time-division multiplex signal receiver 20.
Referring to FIG. 2, an optical time-division multiplex signal OTDM transmitted through an optical fiber 21 is supplied to an all-optical gate 23, wherein the all-optical gate 23 is supplied with an optical clock signal via an optical waveguide 22 and changes a transmittance thereof in response to the optical clock signal. As a result, the optical signal component of the channel corresponding to the optical clock signal is extracted and is outputted to an optical waveguide 24 formed at an output side of the all-optical gate 23. The optical signal component thus extracted is detected by a photodiode 25.
However, in the construction of FIG. 1, there exists a drawback in that, while the response speed of the electro-optic modulator 12 is faster than the response speed of a photodiode, there is a limit and the advantage may be lost in the case the transmission rate of the optical time-division multiplex signal is increased further in future.
In the construction of FIG. 2, the all-optical gate 23 has a response speed sufficient for responding to a high-speed optical time-division multiplex signal. However, such an all-optical gate 23, relying upon the optical-absorption-saturation phenomenon caused by an optical clock signal, requires a strong optical clock signal for on-off driving, and there arises a problem in that a large and bulky construction is necessary.